The invention relates to mechanically formed heat transfer tubes for use in various applications, including boiling and condensing. In submerged chiller refrigerating applications, the outside of the tube is submerged in a refrigerant to be boiled, while the inside conveys liquid, usually water, which is chilled as it gives up its heat to the tube and refrigerant. In condensing applications, the heat transfer is in the opposite direction from boiling applications. In either boiling or condensing, it is desirable to maximize the overall heat transfer coefficient. Also, anytime the efficiency of one tube surface is improved to an extent that the other surface provides the majority of the thermal resistance, it would of course be desirable to attempt to improve the said other surface. The reason for this is that an improvement in the reduction of thermal resistance of either side has the greatest overall benefit when the inside and outside resistances are in balance. Much work has been done to improve the efficiency of heat transfer tubes, and particularly boiling tubes, since it is easier to form enhancements on the outside surface as compared to the inside surface.
Typically, modifications are made to the outside tube surface to produce multiple cavities, openings, or enclosures which function mechanically to permit small vapor bubbles to be formed. The cavities thus produced form nucleation sites where the vapor bubbles tend to form and start to grow in size before they break away from the surface and allow additional liquid to take their vacated space and start all over again to form another bubble. Some examples of prior art patents relating to mechanically produced nucleation sites include Zatell U.S. Pat. No. 3,768,290, Webb U.S. Pat. No. 3,696,861, Campbell et al U.S. Pat. No. 4,040,479, Fujikake U.S. Pat. No. 4,216,826 and Mathur et al U.S. Pat. No. 4,438,807. In each of these patents, the outside surface is finned at some point in the manufacturing process. In the Campbell et al patent the tube is knurled before it is finned so as to produce splits during finning which are much wider than the width of the original knurl grooves and which extend across the width of the fin tips after finning. In the remaining patents, the fins are rolled over or flattened after they are formed so as to produce narrow gaps which overlie the larger cavities or channels defined by the roots of the fins and the sides of adjacent pairs of fins. The Fujikake patent provides an especially efficient outside surface which is produced by finning a plain tube, pressing a plurality of transverse grooves into the tips of the fins in the direction of the tube axis and then pressing down the fin tips to produce a plurality of generally rectangular, wide, thickened head portions which are separated from each other between the fins by a narrow gap which overlies a relatively wide channel in the root area of the fins.
The prior art has also considered the fact that it is not enough to merely improve the heat transfer efficiency of a tube on its boiling side. For example, Withers et al U.S. Pat. No. 3,847,212, which is assigned to a common assignee and incorporated by reference herein, discloses a finned tube with a greatly enhanced internal surface. The enhancement comprises the use of multiple-start internal ridges which have a ridge width to pitch ratio which is preferably in the range of 0.10-0.20. Thus, a longitudinal flat region exists between internal ridges which is substantially longer, in an axial direction, than the width of the ridge. The patentee states that heat transfer efficiency is improved by decreasing the width of the ridge relative to the pitch. Presumably, the efficiency would be expected to drop when the ridges are placed too close to each other since the fluid would tend to flow over the tips and not contact the flat surfaces in between the ridges. This condition would exist because the ridges were located generally transverse to the axis of the tube. Specifically, an angle of 39.degree. from a line normal to the tube axis was disclosed. Obviously, the corresponding angle measured relative to the tube axis would be 51.degree.. Although the Withers et al design balanced the efficiencies of the inner and outer surfaces relatively uniformly, its outer boiling surface was not as efficient as more recent developments such as the surface disclosed by Fujikake. Other tubes with internal ridges are disclosed in the following U.S. patents: Rodgers, U.S. Pat. No. 3,217,799; Theophilos, U.S. Pat. No. 3,457,990; French, U.S. Pat. No. 3,750,709; Rieger, U.S. Pat. No. 3,768,291; Fujie et al, U.S. Pat. No. 4,044,797 and Lord et al U.S. Pat. No. 4,118,944.